The present invention relates to polarization switching in the field of optical fiber communications and, more particularly, to special design of bit precoders.
Performance of optical fiber communications is limited by an optical signal-to-noise ratio and fiber nonlinearity. To enhance the fiber nonlinearity tolerance, polarization switching has been proposed to reduce the nonlinear cross-talk between adjacent symbols [1]. The advantages of polarization switching have been demonstrated in experiments. Polarization switching formats requires special design of bit precoders, such as 2-bit precoders for the differential quaternary phase-shift keying (DQPSK) format. A 2-bit DQPSK precoder may work as illustrated in FIG. 3. So far, however, no particular documents have been reported to implement the polarization switching in the field, because no such 2-bits differential precoder is available. At this moment, no product has been developed to perform 2-bit DQPSK precoding. Commercially available differential DQPSK precoders only perform the precoding scheme on a symbol-by-symbol basis. It is very costly to redesign the whole application-specific integrated circuit (ASIC) chips. A 1-bit DQPSK precoder [2] could be re-designed to perform 2-bit differential encoding, though the cost would be much higher than our proposed structure as follows:
We propose to use a combination of discrete components such as commercial 1-bit DQPSK precoders to construct a 2-bit DQPSK precoder, thus enabling the commercial deployment of polarization switched signals in the DQPSK system. It is the first time that a feasible scheme is proposed to allow polarization switching DQPSK being deployed in an actual optical transmission system.
[1] J. K. Fischer, and K. Petermann, “Performance Analysis of CSRZ-OOK With Pairwise or Pulse-to-Pulse Alternate Polarization”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 24, DEC. 15, 2007
[2] Product Brief, SMI4029, http://www.semtech.com/images/datasheet/smi4029.pdf